I liked it. I liked it a lot.
The final book of the Karla trilogy that started with Tinker, Tailor, Soldier, Spy.
I liked it, because every character has a back story and a motivation. I understood why everyone did everything (at least by the end). I liked the tradecraft and the anecdotal observations of it (see especially chapter 21).
I liked the moral ambivalence of it. Maybe the bad guys were always bad, but had the good guys been corrupted in the long fight?
I liked the prose. Le Carre is at the height of his powers. Fresh, powerful, lyrical...transparent. You don't even notice it. You glide along with it, all in the service of the story...you don't even realize you're reading.
This is better than Tinker, better than The Spy Who Came in From the Cold. That's saying something.